1. Field of the Invention
The present invention relates to a zoom lens system and an image pickup apparatus including the zoom lens system, and more particularly, to a zoom lens system and an image pickup apparatus including the zoom lens system, which are suitable for an image pickup optical system used for a still camera, a video camera, a digital still camera, a TV camera, or a monitoring camera.
2. Description of the Related Art
In recent years, the number of pixels of an image pickup element used for an image pickup apparatus such as a digital camera or a video camera has been increased. An image taking lens system used for the image pickup apparatus including the image pickup element is required to be a zoom lens system in which not only monochromatic aberrations such as spherical aberration and coma but also chromatic aberration are excellently corrected. A high zoom ratio is also required. In particular, a zoom lens system having a long focal length on a telephoto side and a high zoom ratio is required to have an excellently corrected secondary spectrum.
A positive-lead zoom lens system has been known as the zoom lens system having the high zoom ratio. The positive-lead zoom lens system includes, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a rear lens group including a lens unit having a positive refractive power. As a positive-lead zoom lens system in which a lens unit having a positive refractive power is located closest to the object side, there has been known a zoom lens system in which at least four lens units are provided and an anomalous dispersion material is used as a lens material of a first lens unit to excellently correct a secondary spectrum of chromatic aberration (U.S. Pat. No. 6,594,087). With respect to a four-unit zoom lens system including a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power, which are provided in order from the object side, there has been known a zoom lens system using a lens made of a material having low dispersion and anomalous dispersion for the first lens unit (U.S. Pat. No. 7,139,131 and U.S. Pat. No. 6,404,561). With respect to a five-unit zoom lens system including a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, a fourth lens unit having a negative refractive power, and a fifth lens unit having a positive refractive power, which are provided in order from the object side, there has been known a zoom lens system using a lens made of a material having anomalous dispersion for the first lens unit (U.S. Pat. No. 7,304,805 and U.S. Pat. No. 7,505,214). Further, there has been known a zoom lens system in which chromatic aberration is reduced by a replica layer made of a transparent resin having anomalous dispersion (Japanese Patent Application Laid-Open No. 2008-191286).
The positive-lead zoom lens system is relatively easy to realize a high zoom ratio while the entire system is reduced in size. However, the secondary spectrum of axial chromatic aberration increases in a zoom range on the telephoto side. In the four-unit zoom lens system or the five-unit zoom lens system as described above, an incident beam height of the first lens unit is large in a zoom range on the telephoto side, and hence the axial chromatic aberration is generated mainly in the first lens unit. In this case, in order to reduce the secondary spectrum to correct the axial chromatic aberration, it is important to use the first lens unit in which a height of the paraxial ray is large in the zoom range on the telephoto side. In order to reduce the secondary spectrum of the axial chromatic aberration in the first lens unit, it is effective to use a material having low dispersion and anomalous dispersion for a lens included in the first lens unit.
However, a normal optical material having low dispersion and anomalous dispersion has a low refractive index as in a typical case of fluorite. Therefore, when a refractive power of the lens is to be changed to desirably correct the secondary spectrum, it is necessary to significantly adjust a curvature of the lens. The height of the paraxial ray of the first lens unit of the positive-lead zoom lens system is large at the telephoto end, and hence not only the axial chromatic aberration but also spherical aberration and coma are generated more. Thus, when only the optical material having low dispersion and anomalous dispersion or only a resin material having a low refractive index is used, it is difficult to excellently correct chromatic aberration, spherical aberration, and coma at the telephoto end. In order to excellently correct the chromatic aberration, the spherical aberration, and the coma, the number of lenses included in the first lens unit may be increased. However, when the number of lenses of the first lens unit increases, the entire lens length is more likely to be increased.